Dry descaling

ABSTRACT

Dry descaling of hot-rolled, steel strip wherein the strip, having a layer of magnetite on its surface, is heated in a normal furnace atmosphere to at least 1,500*F such that conversion of the magnetite to wustite is effected. Thereafter, the strip is immediately quenched below 600*F by air jets at a rate rapid enough to prevent reconversion of the wustite to magnetite. The wustite scale is subsequently removed by abrasion.

nited States Patent 1191 1111 3,830,023 Turner 7 Aug. 20,1974

1 1 DRY DESCALING 3,699,726 10/1972 Turner 51/322 7 [75] Inventor:Charles A. Turner, Flourtown, Pa.

[73] Assignee: Selas Corporation of America, Dresher, Pa.

Primary ExaminerD0nald G. Kelly [57] ABSTRACT [22] Flledz Oct. 30, 1972I Dry descalmg of hot-rolled, steel stnp wherem the 1211 Appl'[QC-13021067 strip, having a layer of magnetite on its surface, is

heated in a normal furnace atmosphere to at least 52 us. ca. 51/322 Suchthat chhversioh of the magnetite to 51 1111.01 B24b 1/00 the iseffected- Thereafter, the Strip is immediately [58] Field of Search51/322, 323, 324 quenched below 600F y air jets at rate rapid enough toprevent reconversion of the wustite t0 mag- 5 References Cited netite.The wustite scale is subsequently removed by UNITED STATES PATENTSabraslon- 3,166,841 1/1965 Gebhard 51/322 UX 3 Claims, N0 Drawings DRYDESCALING The present invention relates to the descaling of steel andmore particularly to a method of dry descaling of strip metal utilizingheat conversion of the scale followed by a mechanical removal means. I

In the manufacture of hot-rolled strip steel, the

metal, as it leaves the hot rolls is coiled either just I below therolling temperature or at a lower temperature. In either case, a scalein the form of magnetite results on the surface. This scale must beremoved before the strip is used in the hot-rolled condition or is coldreduced to lighter gauges. Various methods of removing the scale arepracticed, the most usual of which is acid pickling. Another method ofscale removal is described in the Patent to Gebhard et al., No.3,166,841, in which the hot-rolled strip is heated in an inertatmosphere to a temperature above lO76F. At this temperature magnetiteconverts to wustite, a more easily removed scale. The strip must then becooled under an inert atmosphere to below the reconversion temperaturebefore the wustite scale is removed by pickling or mechanical action.

Another dry descaling method is disclosed in Turner et al., Patent No.3,699,726, in which the hot-rolled strip is heated in the atmosphere tonot above the Ac transformation temperature or 1400F maximum, but abovelO76F, to convert the magnetite to wustite. The strip is then rapidlycooled below 600F by air jets before being run through breaker rolls andsubjected to abrasive action to remove the scale. This process whilebeing fairly simple and producing a clean strip has the disadvantage ofhardening the strip. The breaker rolls induce a strain that results insubsequent spontaneous age hardening of several points. While for someend uses this is of no consequence, for other uses, such as deepdrawing, the increased hardness is objectionable.

The Gebhard patent and the Turner et al. patents are based uponconsideration of converting the prior scale, as magnetite, to wustite.Aside from satisfying this basic purpose, it has been established thatsub-critical heating for scale conversion accounts for a substantialhardening effect on the steel strip, amounting to an increase ofRockwell B hardness of some 15 to 20 points. This hardening results froman alteration in the iron carbide size and distribution by the heatingto a sub-critical temperature with subsequent fast air cooling and bythe strain-hardening effects of roll breakering and resultant agehardening of the steel at room temperature following this processing.This increase in hardness renders the steel unsuited for subsequentprocessing, but most particularly where the end product is processeddirectly from the hot-rolled and descaled strip.

It is an object of the present invention to provide a method of drydescaling of strip steel which is simpler and less expensive than thosepracticed in the prior art and one in which the forming characteristicsof the steel are not adversely affected when applied to commercialquality and most drawing quality forming applications in the hot-rolledcondition.

Steel strip for drawing purposes is usually coiled at a temperature offrom l350-l400 F involving air cool on the run-out table. This practiceaccounts for a thick, tenacious scale that must be removed before beingdrawn or before cold reduction and annealing to lighter gauge drawingapplications.

The scale that is formed is magnetite which is difficult to remove. Thisscale can be converted, when heated above 1076F to wustite that is moreeasily removed. The wustite, however, is unstable and will revert tomagnetite as the steel is cooled if special precautions are not used. Itis also known that wustite forms a smooth interface with the iron basewhereas the magnetite appears to have finger-like projections thatextend between the grain boundries of the underlying iron, making totalclean-up difficult by any descaling method. 1

The equipment required to practice the present invention is a stripheating furnace of conventional type in which a strip can be heated to1700F in an atmosphere of products of combustion. In addition, there isrequired some mechanical means for removing scale such as brushes orshotblasting apparatus.

In practicing the invention, hot-rolled strip with a scale on itssurface, usually magnetite, is continuously passed through a fuel firedor electric furnace having an atmosphere that can be. ordinary hotproducts of combustion or air. The most efficient heating is obtained,regardless of fuel, when the products of combustion are neutral, but forpurposes of the invention they can be slightly oxidizing or reducing.The furnace is maintained at such a temperature, and the strip is movedtherethrough at such a speed the strip will be heated to a temperatureof approximately I 700F when it leaves the furnace. During the heatingthe magnetite scale is converted to wustite. As the scale converts italso increases microscopically in thickness, sufficient to oxidize belowthe finger-like projections of the magnetite, thus creating asubstantially smooth interface between the wustite and the underlyingmetal.

Immediately after heating, the strip is cooled by blowing jets of airagainst both sides of the strip to cool it below 600F and preferably toambient temperature. This rapid cooling avoids reconversion of wustiteback .to magnetite until the temperature is below that at which thereconversion takes place. Thus, the strip is covered with a coating ofeasily removed scale in the form of wustite.

The strip is then moved directly to and through abrading apparatus toremove the scale. Strip cleaned in this fashion shows a clean, brightand smooth surface, and is immediately ready for subsequent processmg.

Normally, when hot-rolled strip is coiled hot, from 13501400F, the stripaccording to the invention, is heated to about 1700F. If the hot-rolledstrip is coiled at a lower temperature of from l000l 100F, the scaleconverting heating can be done at a lower temperature, in the rangel500-l550F. ln either event, however, the strip is heated to well abovethe lower critical temperature in order both to convert the scale andincrease scale thickness sufficiently to obtain a smooth interface. Thestrip can, of course, be heated to a temperature above 1700F if desired.This increase, however, does not make the scale any easier to remove.

Ordinarily, the strip will be moved continuously through the furnace,air quench and abrading apparatus. It will be obvious, however, that theprocess is the same if the strip is heated and quenched, then coiled,with the abrading being done at a later time just prior to being used.

lf defluting of the strip is desired it can be passed through a set ofleveling rollers after the quench. Such rollers will have no appreciableeffecton the characteristics of the strip, but will tend to crack thescale and thus facilitate its removal.

While in accordance with the provisions of the Statutes l haveillustrated and described the best form of 5 embodiment of my inventionnow known to me, it will be apparent to those skilled in the art thatchanges may be made in the form of the apparatus disclosed withoutdeparting from the spirit and scope of the invention set forth in theappended claims, and that in some cases certain features of my inventionmay be used to advantage without a corresponding use of other features.

What is claimed is:

l. The method of descaling a hot-rolled steel strip having a layer ofmagnetite on its surface which comprises heating the strip in a furnacein an normal furnace atmosphere to a temperature of at least l500 Fthereby to convert the magnetite to wustite and increase the thicknessthereof, immediately thereafter quenching the strip to below 600F bydirecting air jets against both sides thereof at a rapid enough rate toprevent reconversion of the wustite to magnetite, and subsequentlyremoving the wustite scale from the surface of the strip by abrasion.

2. The method of claim 1 in which the strip is heated to a temperatureof approximately l700F.

3. The method of claim 1 in which heating of the strip is continued toincrease the scale in thickness sufficiently to produce a substantiallysmooth interface between the scale and the underlying metal.

2. The method of claim 1 in which the strip is heated to a temperatureof approximately 1700*F.
 3. The method of claim 1 in which heating ofthe strip is continued to increase the scale in thickness sufficientlyto produce a substantially smooth interface between the scale and theunderlying metal.